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Chapter 8
Mendel & Inheritance
Genes, alleles & inheritance
• Genes – Nucleotide sequence
that code for a specific trait.
– The expressed trait is called a
character or Phenotype
• Trait – Manifestation of genes
• Genome – A complete set of
ALL the genes in an organism
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Genes, alleles & inheritance
• Allele – Copy of a gene
– Copies carried by an single
individual called genotype
• Locus – Physical location of a
gene on a strand of DNA
• Chromosome – Organized
group of genes
Gene
A
B
C
D
– Part of a genome
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Terminology
• Homozygous –
homologous copies of
a gene are the same
• Heterozygous –
homologous copies of
a gene are different
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Gene mutations & traits
• How do new alleles arise?
• Mutation:
• Different allele  different trait
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Trait Inheritance
• Heredity – the
transmission of traits
from parents to
children
– Inherit ½ from each
parents
• Genes program specific
traits
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Who is Gregor Mendel?
• German monk 1822-1884
• Most monks were committed to theology and the
study of nature
• Mendel started
on his family farm
studying bees and
physics
• Interested in how
characteristics
formed
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Mendel’s pea plants
Why peas?
• Fast Growing
– Easy to grow in greenhouse
or garden
– Easy to maintain large
numbers
• Easy to control mating
• Many clearly identified
traits
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Mendel’s
experiments
• Parental (P)
Generation
– Collected stamen
(male organs)
– Used to fertilize a
stigma (female organ)
• Collected offspring
(seeds)
– Grew plants from
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Mendel’s experiments
• Collected two of his new
plants (F1 Generation)
• Repeated fertilization
process
– looked at the resulting
plants (F2 Generation)
• Repeated experiment a
Thousands of times
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Predicting
outcomes
• Mendel found
there were two
types of traits:
• Dominant
• Recessive
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Punnett Squares
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Coin Toss
• Segregation pattern
of alleles is a chance
event
– Like a coin toss
• We can predict the
probability of
– a coin landing heads
up
– a coin landing tails up
• Flip two coins and you
get a 1:2:1 ratio
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Phenotypic & Genotypic Results
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Mendel’s law #1
• Segregation of Chromosomes
• The two homologus copies of a
gene end up in different
gametes
– Meiosis
• Therefore, possible offspring
genotypes predicted using
mother and fathers genotypes
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For Example
• Can you Roll your tongue?
• T= Yes; t= No
TT? Tt? Tt?
• Dominant genes:
• Recessive genes:
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For Example
• You have ____ copies of the
tongue rolling gene.
• Your Genotype:
• Homozygous:
• Heterozygous:
Alleles
T= Yes; t= No
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Diagramming inheritance
• Draw a punnett square to predict the phenotypes
TT
tt
tt
Tt
Tt
Tt
t
t
Tt
Tt
Tt
T
T
t
T
Tt
t
T
Tt
TT
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Mendel’s pea plants
• 3:1 Pattern of inheritance
found in a variety of traits
– Called Mendelian Genetics
• Parent pea
plants pass on
characteristics
to offspring
• Published his
results in
1866
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Multi-trait cross
• Dihybrid cross
• Used to determine
the ration of two
genes
• Can determine if
genes are connected
in some way
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Di-hybrid cross
• Human
eye
color
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Mendel’s law #2
• Independent Assortment
• Chromosomes are
randomly & independently
assigned to gametes
• Therefore traits on different
chromosomes are inherited
independently
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Independent Assortment
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Incomplete Dominance
• Two alleles produce
different phenotypes
– Homozygous genotype
• When these two alleles are
together produce a third
phenotype
– intermediate
Heterozygous genotype  Intermediate phenotype
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Incomplete dominance
Heterozygous genotype  Intermediate phenotype
HcHc
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4902/6613115/IMG_0643-vi.jpg
HcHs
http://www.cosmogirl.com/cm/cosmogirl/images/coral
-eyeshadow-gtl0606-240x312.jpg
HsHs
http://www.annabelleswigs.co.uk/ima
ges/clips_STW_24_col130.JPG
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Human Blood Types (Co-dominance)
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Continuous variation
• Mendelian trait
– Each trait controlled by a single gene
• Continuous variation (polygenic trait)
– Multiple genes of small effect
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Continuous variation
• Human
Skin
Color
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Epigenetics
• Environment can affect phenotype
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Questions:
• Why do some conditions
“run in the family”?
• Why are some children
or some generations not
affected?
• What are mendelian
traits?
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Questions:
• Dominant traits
• Genetic recombination
(be able to explain this)
• Mendelian Traits show
typical
dominant/recessive
expression, based on the
combination of two
alleles
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Questions:
• Be able to do simple
mendelian crosses
– PP x pp
– Pp x pp
– Pp x Pp
• What is:
– Incomplete dominance?
– Co-dominance?
– Polygenic Traits?
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